1. Field of the Invention
This invention relates to the process of designing and fabricating semiconductor chips. More specifically, the invention relates to a method and an apparatus for performing model-based optical proximity correction (OPC), wherein feedback is used to expedite the convergence of the OPC process.
2. Related Art
As integration densities on a semiconductor chip continue to increase at an exponential rate, it is becoming progressively harder to deal with optical effects that arise during the optical lithography process involved in manufacturing the semiconductor chips. These optical effects can cause unwanted distortions in the printed layout that are generated by the optical lithography process.
To remedy this problem, a layout is often subjected to a model-based optical proximity correction (OPC) process, which adjusts the layout to compensate for optical effects. (Although the term “optical proximity correction” is used in this specification, more generally the term as used herein refers to correction for any specified proximity effects, e.g. optical, micro-loading, etch, resist, phase shift mask (PSM), etc.) These adjustments are made based upon results of model-based simulations of the printed layout. During this model-based OPC process, edges in the layout are divided into segments, and each segment is adjusted with a negative or a positive bias based upon a deviation between the desired layout and the simulated layout. The model-based OPC process then re-simulates the layout and measures the deviation of the resulting output pattern from the expected position, and re-adjusts the edge biases based on this deviation.
In a model-based OPC process, a number of iterations of such edge adjustments are necessary for the process to converge on an acceptable result. This is because segments interact with neighboring segments, so adjustments to a given segment can affect the final printed layout for neighboring segments. Furthermore, as pattern features become smaller, different correction areas tend to correct at different rates, and these differing correction rates can cause the OPC process to oscillate. Consequently, convergence to a final solution can require more iterations.
It is important to minimize the number of OPC iterations, because performing a large number of iterations takes a great deal of time, which can greatly increase the cost of generating the output mask pattern. Hence, what is needed is a method and an apparatus for expediting convergence in a model-based OPC process.